This invention relates to a photoelectric measuring system of the type which comprises a substrate, an integrated optical circuit on the substrate, and at least one diffraction grid shiftable with respect to the substrate in a direction transverse to a beam axis, wherein the integrated optical circuit includes two optical waveguides, two waveguide horns, each configured to couple a respective diffracted component beam from the diffraction grid to a respective one of the optical waveguides, a coupler configured to receive the diffracted partial beams from the optical waveguides and to bring the diffracted component beams into interference to form a combined beam, and at least one detector positioned to respond to the combined beam and to generate at least one electrical signal in response to the combined beam.
In the last few years there has been substantial progress in the development of velocity and position measuring systems. Such measuring systems have been developed for both process and testing applications, and they utilize optical, magnetic, and other sensing techniques in combination with suitable electronic circuits. Measuring devices in which light is used for the measuring standard are known as optical interference measuring devices, and in these devices it is the wavelength of monochromatic light such as laser light that is used as the reference magnitude. The accuracy of these measuring devices is suitable for current industrial technology, but in many cases the obtainable high precision of measurement requires a large economic expenditure.
Measuring devices have been proposed in which an illuminating system and a receiver are arranged on a common conductor plate. See for example German DE-OS 34 27 047.
In many applications such as those involving machine tools, for example, a measuring device is required having a mean accuracy which lies between the high accuracy of optical interference measuring devices described above and the lower accuracy of magnetic measuring systems. For these applications an optical diffraction grid can be used, and in such systems the grid constant of the diffraction grid typically is on the order of a few microns. Such measuring devices represent a compromise between high accuracy and acceptable expenses. Such devices are described for example in German DE-OS 33 16 144 and in Japanese unexamined patent specification JP-OS 59-164 914.
In such devices the diffraction grid itself defines the reference magnitude. A diffraction grid is made up of very thin grid lines which lie closely adjacent to one another on a glass or metal plate and are formed by any of a variety of mechanical processing techniques, photolithographic techniques, electron beam lithography techniques, or the like. Such systems also include the following: a light source that emits monochromatic light (for example laser light), two reflector mirrors, and a detector positioned on the side opposite the light source to receive interference light. The light beam emitted by the light source is diffracted by the diffraction grid and passed. A light beam diffracted by the diffraction grid includes diffracted light (a diffraction component beam) of the N-th order. Under the influence of the diffraction grid there is obtained in the wavefront of the light a value N.rho., the product of the order number and the phase. The light beam that runs rectilinearly through the diffraction grid contains no phase information. The two light beams are reflected from the reflector mirrors and return along the outward path to enter the diffraction anew and to be diffracted and passed through by the diffraction grid. The passed light of the one light beam and the diffraction light of N-th order the other light beam are spatially selected so as to interfere with one another and enter a detector. In the diffracted light of N-th order of the second light beam, the value -N.rho. of the opposite sign is obtained through the phase of the diffraction grid, while in the passed light of the first light beam only the previously arisen phase N.rho. is present. For this reason, the interference light corresponds to 2N.rho. which corresponds to twice the amount of the phase of the diffraction grid. If therefore one assumes that the diffraction grid moves with respect to another part of the optical system (for example with respect to the light source and the reflector mirrors) then the interference light moves over two N periods when the diffraction grid moves over one period.
In another known arrangement, the light beam emitted from the light source is diffracted by the diffraction grid and component beams of the same order with different signs overlap and interfere with one another, as a semipermiable mirror or the like is provided before the light enters the detector. In this way by reason of the phase of the diffraction grid in the diffracted light beams, there is obtained the magnitudes N.rho. and -N.rho., in which N is the diffraction order number. Therefore, the interference light is obtained with a value 2N.rho., an amount equal to twice the phase of the diffraction grid. If, therefore, one then again assumes that the diffraction grid and another part of the optical system are moved relative to one another, as explained above, then the interference light moves 2N periods while the diffraction grid moves over one period.
In order to make it possible to accommodate the optical system described above in a small space, it is necessary to compensate for the angle of the light beams with respect to the diffraction grid. If in this case, however, the relative position of the optical system with respect to the diffraction grid is shifted in the direction of the grid lines of the diffraction grid, there occurs a phase change that is similar to the phase change that arises when relative movement occurs perpendicularly to the plane of the diffraction grid. For this reason, the measuring accuracy is relatively low. If the light beam is incident on the diffraction grid perpendicularly, then the above explained disadvantage is avoided, but the optical system becomes extensive so that at a relatively large amount of space must be provided.
U.S. patent application Ser. No. 07/077,190 (corresponding to German Patent Application P 36 25 327.8-82) describes a position measuring system which is simple in construction and yet which substantially avoids undesirable environmental influences on the measuring device so that dependable operation is provided. The device described in this patent application provides the advantage of a compact construction with integration capability and security from interference from environmental influences.
The present invention is directed to an improved measuring system of the general type described above which forms components in such a way as to yield a higher degree of integration.